J. Neurochem. (2010) 114, 565–575. Abstract Glutamate is the predominant excitatory neurotransmitter in the mammalian central nervous system and extracellular glutamate levels are controlled by a family of transporters known as excitatory amino acid transporters (EAATs). The EAATs transport glutamate and aspartate with similar micromolar affinities and this transport is coupled to the movement of Na+, K+, and H+. The crystal structure of a prokaryotic homologue of the EAATs, aspartate transporter from Pyrococcus horokoshii (GltPh), has yielded important insights into the architecture of this transporter family. GltPh is a Na+‐dependent transporter that has significantly higher affinity for aspartate over glutamate and is not coupled to H+ or K+. The highly conserved carboxy‐terminal domains of the EAATs and GltPh contain the substrate and ion binding sites, however, there are a couple of striking differences in this region that we have investigated to better understand the transport mechanism. An arginine residue is in close proximity to the substrate binding site of both GltPh and the EAATs, but is located in transmembrane domain (TM) 8 in the EAATs and hairpin loop 1 (HP1) of GltPh. Here we report that the position of this arginine residue can explain some of the functional differences observed between the EAATs and GltPh. Moving the arginine residue from TM8 to HP1 in EAAT1 results in a transporter that has significantly increased affinity for both glutamate and aspartate and is K+ independent. Conversely, moving the arginine residue from HP1 to TM8 in GltPh results in a transporter that has reduced affinity for aspartate.